1. Field of the Invention
The present invention relates generally to a magnetic sensor apparatus, and more particularly to a magnetic sensor apparatus that can detect very weak magnetic fields.
2. Discussion of Background
The methods for detecting magnetic fields in the known art include the parallel flux gate method and the cross flux gate method. In the parallel flux gate method, which employs an exciting winding and a detection winding provided at a toroidal core or the like, the direction of a magnetic field resulting from magnetic excitation and the direction of the magnetic field to be detected are parallel.
In the cross flux gate method, a detection winding is provided around an amorphous magnetic metal alloy wire or foil and a current is directly supplied to the amorphous magnetic metal alloy wire or the foil to achieve an exciting function. In this method, the direction of the magnetic field resulting from magnetic excitation and the direction of the magnetic field to be detected extend orthogonally to each other.
In both of the methods described above, the magnetic permeability of a magnetic body is changed through magnetic excitation to generate an induced voltage, which is in proportion to an external magnetic filed generated at the detection winding. However, in the former method, since the impedance at the exciting winding is high, the excitation speed cannot be increased. Thus, there are problems in that the sensitivity is poor and that the apparatus will become expensive to produce.
In the case of a sensor employing an amorphous magnetic metal which is an example of the latter method, miniaturization can be achieved easily while achieving a high degree of sensitivity. However, since the relative coercivity is great, it is difficult to perform detection at a zero magnetic field. Thus, a bias magnetic field is required as a means for achieving detection at the zero magnetic field. However, the stability of the sensor output at the time of bias magnetic field application is problematic since it is subject to the influence of temperature-related sensitivity fluctuations and drift.
An example of a magnetic sensor element employing the cross flux gate method is disclosed in Japanese Patent No. 2,617,498, and Japanese Unexamined Patent Publication No. 166437/1997 discloses an example of a detection circuit employing this magnetic sensor element.
In the magnetic sensor element adopting the cross flux gate method, a detection winding is wound around a conductive magnetic body having a high degree of magnetic permeability, which is formed in a linear shape, a rod shape, a band shape or the like and is provided with a linear portion in the lengthwise direction, and the magnetic body is excited to a point close to saturation by supplying a pulse current in the direction of the length of the magnetic body to greatly change the magnetic permeability .mu. of the magnetic body. A voltage V induced at the detection winding at this point is determined through the formula presented below and is in proportion to an external magnetic field. Consequently, the external magnetic field can be detected based upon the voltage V determined through the following formula: EQU V=d(.mu..multidot.H.multidot.S)/dt;
wherein
".mu." represents magnetic permeability of the magnetic body itself; PA1 "H" represents external magnetic field; and PA1 "S" represents the cross sectional area of the magnetic body.
However, the magnetic field generated by a pulse current, which extends orthogonally to the direction in which the detection winding is transversed does not generate an induced voltage unless another magnetic cross flux is present. The induced voltage V increases as the external magnetic cross flux (magnetic field) becomes larger or the magnetic permeability of the magnetic body itself becomes higher, and as the pulse that is applied is steeper. Conductive magnetic bodies suited for this application may be constituted of cobalt amorphous magnetic metal alloy wires, foils or the like.
FIG. 3 shows an example of a magnetic sensor element adopting the cross flux gate method. A magnetic sensor element S is achieved by etching conductive amorphous magnetic alloy foil formed in a band shape which is pasted onto an insulating substrate 1 constituted of an epoxy resin or the like to constitute a magnetic body M with a specific pattern, with a wire loop wound around the magnetic body M to constitute a detection winding Wd.
The dimensions of the magnetic body M having a linear portion in the lengthwise direction may be, for instance, 5 mm (width).times.15 mm (length). The two ends of the magnetic body M are each connected to an excitation terminal 2 secured to the insulating substrate 1, whereas the two lead-out ends of the detection winding Wd are each connected to a detection terminal 3 secured to the insulating substrate 1.
Now, since a magnetic body, by nature, demonstrates hysteresis, no change occurs in the magnetic flux and no output manifests at the detection winding unless there is a magnetic field that exceeds its coercivity. Thus, in order to detect a very weak magnetic field such as geomagnetism with the magnetic sensor element S described above as an analog signal, a DC bias magnetic field achieving a specific size is applied in the prior art to obtain a signal output. In addition, when good linearity is to be achieved, the so-called feedback method is employed. Namely, when an external magnetic field increases, the signal corresponding to the increase is amplified and control is implemented to reduce the DC bias magnetic field in the reverse direction. If the degree of amplification at the amplifier circuit is set to a value as high as possible, the synthesized magnetic field applied to the magnetic sensor element in a feedback equilibrium state achieves a value in the vicinity of the DC bias magnetic field at all times. In other words, a signal using this DC bias magnetic field as its operating point is output. However, due to factors such as inconsistency of the magnetic characteristics, temperature-related fluctuations, distortion-related fluctuations and drift in the magnetic permeability which is represented as the rate of change in the B-H curve of the magnetic body, the output signal using the DC bias magnetic field as its operating point tends to fluctuate easily and, therefore, it is difficult to obtain a stable output signal.